Neptune: Rocking The Dreidel

byTammy PlotneronJuly 1, 2011

When it come to making your head spin, Jupiter revolves on its axis in less than 10 hours. Up until now, it was the only gas planet in our solar system that had an accurate spin measurement. But grab your top and cut it loose, because University of Arizona planetary scientist Erich Karkoschka has clocked Neptune cruising around at a cool 15 hours, 57 minutes and 59 seconds.

“The rotational period of a planet is one of its fundamental properties,” said Karkoschka, a senior staff scientist at the UA’s Lunar and Planetary Laboratory. “Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets.”

Like spinning gelatin, the gas giants – Jupiter, Saturn, Uranus and Neptune – don’t behave in an easy to study manner. By nature they deform as they rotate, making accurate estimates difficult to pin down.

“If you looked at Earth from space, you’d see mountains and other features on the ground rotating with great regularity, but if you looked at the clouds, they wouldn’t because the winds change all the time,” Karkoschka explained. “If you look at the giant planets, you don’t see a surface, just a thick cloudy atmosphere.”

Of course, 350 years ago Giovanni Cassini was able to estimate Jupiter’s rotation by observing the Great Red Spot – an atmospheric condition. Neptune has observable atmospheric conditions, too… But they’re just a bit more transitory. “On Neptune, all you see is moving clouds and features in the planet’s atmosphere. Some move faster, some move slower, some accelerate, but you really don’t know what the rotational period is, if there even is some solid inner core that is rotating.”

Roughly 60 years ago astronomers discovered Jupiter gave out radio signals. These signals originated from its magnetic field generated by the spinning inner core. Unfortunately signals of this type from the outer planets were simply lost in space before they could be detected from here on Earth. “The only way to measure radio waves is to send spacecraft to those planets,” Karkoschka said. “When Voyager 1 and 2 flew past Saturn, they found radio signals and clocked them at exactly 10.66 hours, and they found radio signals for Uranus and Neptune as well. So based on those radio signals, we thought we knew the rotation periods of those planets.”

In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka)

Using the data from the Voyager probes, Karkoschka went to work studying rotation periods and combined it with available images of Neptune from the Hubble Space Telescope archive. Like Cassini’s work, he carefully studied atmospheric features in hundreds upon hundreds of photographs taken over a time sequence… a period of 20 years. He realized an observer watching the massive planet turn from a fixed spot in space would see these features appear exactly every 15.9663 hours, with less than a few seconds of variation. This led him to surmise a hidden interior feature on Neptune drives the mechanism that creates the atmospheric signature.

“So I dug up the images of Neptune that Voyager took in 1989, which have better resolution than the Hubble images, to see whether I could find anything else in the vicinity of those two features. I discovered six more features that rotate with the same speed, but they were too faint to be visible with the Hubble Space Telescope, and visible to Voyager only for a few months, so we wouldn’t know if the rotational period was accurate to the six digits. But they were really connected. So now we have eight features that are locked together on one planet, and that is really exciting.”

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

It seems to me that, if there is no solid surface, then the only accurate way to measure rotation would be to average a set of measurements. They will all be accurate and they will all be individually wrong. Until and unless we can determine a solid core, we can’t be more accurate than this. Just thinking out loud.

Jeffrey- I did read the article, here and at elsewhere, but that isn’t exactly qualify as ‘determining a solid core’. It may be there, in fact, likely is there, but, if it isn’t, I feel my statement stands.

Averaged how? Did you see the video, and notice the graph, rotation varies. What Karkoska did is, unless I’m mistaken, tantamount to autocorrelation of features. It seems to have resolved in some extremely stable signal signatures.

Conversely, even if you averaged over more features, you would still have to explain the small drift of the ‘autocorrelation’ peaks.

Maybe I don’t understand what you are saying here (determined by observation or determined as in mechanism?), but the article describes that this is an observational possibility and in fact was done – and came up short both as observational technique and explanation of rotation.

Visual signatures were more stable. And of course we expect the massive dog of planet mass momenta to wag the slight tail of magnetic field momenta instead of the reverse.

From the linked press release:

“But when the Cassini probe arrived at Saturn 15 years later, its sensors detected its radio period had changed by about 1 percent. Karkoschka explained that because of its large mass, it was impossible for Saturn to incur that much change in its rotation over such a short time.

“Because the gas planets are so big, they have enough angular momentum to keep them spinning at pretty much the same rate for billions of years,” he said. “So something strange was going on.”

Even more puzzling was Cassini’s later discovery that Saturn’s northern and southern hemispheres appear to be rotating at different speeds.

“That’s when we realized the magnetic field is not like clockwork but slipping,” Karkoschka said. “The interior is rotating and drags the magnetic field along, but because of the solar wind or other, unknown influences, the magnetic field cannot keep up with respect to the planet’s core and lags behind.””